Generally, a variety of electromagnetic waves are emitted from electronic equipment including household electrical appliances, mobile phones, personal computers, and televisions. A strong electromagnetic wave is also emitted from flat panel displays such as a plasma display panel and a liquid crystal television among significantly increasing digital household electrical appliances, and its influence on the human health is also concerned. People observe the images of such displays within a distance relatively close to these displays and for long time in some cases. By doing so, an electromagnetic interference film suppressing these electromagnetic waves is required and the development thereof has been intensively investigated.
In electromagnetic interference films known at present, various methods for suppressing these electromagnetic waves are adopted. For example, a method, in which copper foil is bonded to a polyester film, a regular mesh shape is patterned by photolithography, and etching is performed in such a way that the copper foil becomes a network configuration to prepare a conductive film in which a conductive portion is made from copper and has a network structure, is disclosed in Japanese Unexamined Patent Publication No. 2001-210988 (page 1, claims etc.).
As another method, a method of fabricating a conductive film by printing a composition containing a nucleic agent for plating such as palladium on a film such as polyester in a regular form of network by a printing method and further by applying nonelectrolytic copper plating to a layer consisting of the composition in the form of network is disclosed in Japanese Unexamined Patent Publication No. 2002-185184 (page 1, claims etc,).
As further method, a method of fabricating a conductive film by bonding a knit of metal fiber woven in length and width onto a polyester film or the like is disclosed in Japanese Unexamined Patent Publication No. 11-119672 (page 1, claims etc.).
Further, a conductive body in the form of network which is obtained by initiating micro-cracks in the random form of network and by filling the micro-cracks with a conductive material is disclosed in Japanese Unexamined Patent Publication No. 2004-228057 (page 1, claims etc.), a transparent conductive film provided with a layer in which metal fine particles coagulate in the random form of network is disclosed in Japanese Unexamined Patent Publication No. 2003-71976 (page 1, claims etc.), transparent conductive film provided with a layer containing silver colloidal particles is disclosed in Japanese Unexamined Patent Publication No. 2001-60416 (page 1, claims etc.), and a transparent conductive layer having a fine network structure using silver is disclosed in Japanese Unexamined Patent Publication No. 10-312715 (page 1, claims etc.).
However, there are the following problems in the prior art described above.
A method for etching copper foil is an excellent method for attaining a highly precise network structure, but it is generally low in yields in the steps of bonding copper foil, photolithography, and etching and consequently the yield of a final product becomes low. Further, since a hazardous waste fluid is produced in the etching step, safety and environmental precautions are required. Further, when the copper foil is used as a material, there is also a problem that much of the copper foil is eluted through etching to generate a waste fluid and therefore an amount of a material to be recovered is large.
A method of producing a conductive film by a nonelectrolytic plating process has a feature that an amount of metal waste fluid to be treated is small since metal is used for only required portion. But, since it is difficult to form a pattern of more fine line in the step of printing a mesh form, which is a first step, it is consequently difficult to obtain a conductive film having a conductive portion of a fine line.
A method of bonding metal fiber is a very simple method and readily attains the effect of shielding an electromagnetic wave, but it has a problem that it is difficult to make the network of a conductive portion fine.
The other large problem in technologies described above is a moiré phenomenon occurring when the conductive film is used as an electromagnetic interference film at the surface of a display. The moiré phenomenon is a phenomenon of a fleck pattern in stripe form generated when patterns in which dots or lines are geometrically regularly distributed are superimposed. As for the flat panel display, a moiré in stripe form tends to arise on a screen. This moiré phenomenon arises by an interaction between the pattern of display devices of respective RGB colors of a display main body and the regular configuration of the network of the electromagnetic interference film placed in front of the display main body. These are arrayed regularly to each other, and particularly the display device cannot be changed from a regular configuration to an irregular configuration.
It could be helpful to provide an excellent conductive film which is easy to produce and hardly causes a moiré phenomenon when it is applied to flat panel displays as an electromagnetic interference film.